Following on earlier
work done in this lab on fixed-wing aircraft
wake vortices, we’re now studying the wake vortex structure generated
by rotorcraft. While the wake system of fixed-wing aircraft is a highly
complicated problem, understanding the dynamics of helicopter wake
vortices is especially challenging due to their dependence not only on
the blade geometry and loading, but also on the aircraft’s operational
state (i.e., hovering, climbing, descending, or maneuvering). Our work
is primarily experimental, as we use thrust measurements (from strain
gages), flow visualization techniques
(injecting dye and air bubbles into the flow from the blade tips) and
Particle Image Velocimetry (PIV) to study the wake structure and
dynamics of small model rotors.

Our initial work
involved testing our model rotor in a stationary
water tank -- i.e. simulating a hovering rotor. The flow
visualization and PIV results from these tests have allowed us to gain
a better understanding of the evolution of rotor wakes.
Our primary interest, however, is on the dynamics of
descending rotors. When the descent velocity of a rotor approximately
matches its wake's velocity, the helical wake tends to roll up into a
thick vortex ring that remains near the rotor plane and interferes with
the rotor's inflow. This is known as vortex ring state (VRS) in
helicopter lingo. For reasons that are currently unknown, the vortex
ring that forms has a tendency to periodically detach from the rotor
and convect away. This formation/detachment process can lead to severe
loading fluctuations that can catastrophically impact the performance
of the rotorcraft. The focus of our more advanced work is on the VRS
process, and for this we do our model tests in a 70m long water
towing tank.

Setup

In this
experiment, a three-bladed, 10" diameter rotor with manually-adjustable
blades is used. Each of the carbon-fiber blades has a small tube
embedded in it that allows air bubbles or dye to be leaked from the
tips as a means of marking the tip-vortex cores. The blade airfoils are
ARA-D 10 and the blades have a root-to-tip twist of 5° - relatively low compared to typical
rotorcraft blades.

The
entire model assembly is shown below in Figure 2. The
rotor is driven by a digitally-controlled microstepper motor (25,000
pulses/revolution). The one-inch thick mounting plate beneath the motor
is instrumented with strain gages that allow us to measure the rotor's
thrust during testing.

Figure 3 shows
the full helical wake structure in three dimensions. For the sake of
clarity, only one rotor is injecting air bubbles into the flow.
While air bubbles do an excellent job of showing the details of the
vortex filament structure in the near-wake region of the rotor, they
float to the surface shortly thereafter and so do not show the far
downstream region of the wake.

Figure 3.
Single-rotor air bubble run at 4 rev/s with volumetric illumination.

Figure 4 shows an image
taken using sodium fluorescent dye (being leaked by all
three rotor
blades). A vertical laser sheet is used for illumination here, aligned
with the axis of the rotor. This view provides us with a view of a
single cross-sectional slice of the flow -- thus we clearly see the
vortex cores at the top and bottom of the wake, but none of the helical
structure seen in Figure 3. The smoky region at the left is from dye
emitted
during a previous run. (The tank must be drained and re-filled
frequently during dye testing for best results.)